Computer systems use one or more busses for communicating information between computer components. A processor, for example, communicates with memory, input/output devices, etc. through one or more busses that may comprise a bus hierarchy. The choice of bus architecture is determined by a number of considerations including active/inactive power consumption, ability to reconfigure components (e.g., add, move, remove components), support for multiple voltages, and signal termination.
These busses permit communication of data, address, and control signals between various bus agents. The bus architecture typically includes some feature to permit maintaining or transitioning individual lines to a pre-determined state so that the bus signal lines do not float to indeterminate levels. Floating bus lines can undesirably introduce noise onto the bus communications. In addition, floating bus lines can result in excessive or even destructive power consumption levels for some architectures.
Terminators are used to couple the bus line to a termination supply level. The terminator permits maintaining or moving the bus lines to determinate levels instead of floating. The terminator also serves to diminish reflections at the ends of the bus.
The bus agents should have the same “trip point” such that they identically interpret the state associated with a given bus level. Preferably the trip point is located at the center of the possible bus signal swing for integrity of the signal detection process. The trip point for each bus agent is typically set by voltage dividers or junction voltage drops relative to another supply level on that bus agent.
Although the bus is expected to be driven to one of the same two levels regardless of whether the bus agent is terminated or non-terminated, terminated and non-terminated bus agents experience different voltage drops which shifts the trip point for terminated bus agents relative to the trip point for non-terminated bus agents. The effective window of signal swing for which terminated and non-terminated bus agents properly interpret the bus state is thus narrowed as a result of the variation in trip points.